Hold-downs are generally used to retain electrical connectors on a mounting substrate such as PCBs. Many types of hold-downs can be used to secure the components together. For example, if the use of solder is undesired, the hold-down used may be a rivet or nut and bolt combination. These forms of hold-downs are necessary when the interconnection needs to minimize lateral (X and Y plane) movement relative to the interconnection. Hold-downs may also need to resist unwanted vertical (Z plane) movement such as from mating and unmating forces. Rivets and nut and bolt combinations may be adequate but are expensive and time consuming to manufacture or assemble.
Another typical conventional method of mounting an electrical connector to a printed circuit board is by means of a post provided integrally with the lower surface of the insulating housing of the electrical connector. For example, when the insulating housing is comprised of a plastic material, the conventional hold-down device consists of a post, integrally formed from the plastic housing. The hold-down function is achieved by an interference fit between the post and a corresponding insert opening on the printed circuit board. The connector must be pressed downward so that the plastic post fits into a corresponding opening in the printed circuit board in order to secure the connector on the printed circuit board. Although the conventional interference fit hold-down device can mount an electrical connector tightly on the printed circuit board, such an interference fit plastic post may present problems such as those discussed below.
Among the drawbacks of an interference fit device is the limited dimensional tolerance allowed between the post and the corresponding aperture on the printed circuit board. The problem of maintaining the precisely allowable tolerance between the post and opening is such that even modest variation in the dimension of post or insert opening may cause insufficient retention capability and may result in an unreliable electrical connection between the connector and printed circuit board. Even a slight dimensional variation also may result in excessive interference between the post and opening. This makes it difficult if not impossible to insert the post into the printed circuit board without danger of breaking off or fracturing the post and thereby rendering the entire connector useless.
In order to avoid the foregoing problem, it is necessary to impose significant manufacturing constraints to prevent dimensional variations from occurring during the manufacturing process. This in turn has the disadvantage of significantly increasing the difficulty and cost of manufacturing.
Another disadvantage of a typical interference fit hold-down device arises from the dissimilar thermal expansion coefficients between the integrally molded plastic mounting post and the printed circuit board. When the thermal expansion coefficients of the plastic mounting post and the printed circuit board are sufficiently different, the insert opening on the printed circuit board may apply a force to the free end of the post and cause fracturing or failure of the mounting post.
Another problem of a conventional hold-down device molded from plastic material is that plastic is easily scratched or otherwise subject to abrasion. Also, the conventional hold-down post tends to crack easily during an inappropriate insert-extract operation due to the lack of flexibility of the plastic or the insulating material.
Moreover, conventional hold-downs are typically driven into the connector housing. This fixed positioning can lead to non-coplanarity between the hold-down and the contacts in the electrical connector such as when thermal cycling of the connector occurs, for example during the reflow process which secures the connector to the printed circuit board.
What is needed therefore is a mounting means or hold-down device for securing an electrical connector to a printed circuit board which is not subject to breaking, fracturing or other structural failure, and can float freely in the connector to provide coplanarity between the hold-down and the connector contacts to compensate for any twisting in the housing caused, for example, by thermal cycling.